Process for injection molding of plastic materials

ABSTRACT

A method and apparatus for injection molding of plastic material into a cavity of a mold by means of at least one injector whose opening and closing occurs through an electronic control unit and wherein an undesired opening of the mold caused by any overpressure of the plastic material injected into the cavity of the mold is detected by means of transducers and corrected by means of the electronic control unit. The transducers are arranged outside the cavity of the mold.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Italian Patent Application No.102018000002639 filed Feb. 13, 2018. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention regards the injection molding of plastic materialsinto the cavity of a mold by means of at least one injector whoseopening and closing are operated by means of an electronicservo-control.

STATE OF THE ART

Over the years, the Applicant has designed and developed an electronicinjector control system, referred to as Flexflow®, which guarantees aprecise control of the flow of the injected material during the moldingprocess. In particular the actuator of the or of each injector,typically though not necessarily consisting of a rotary electric motor,is electronically controlled so as to adjust one or more among theposition, speed and acceleration of the pin valve in a programmablemanner, so as to adjust the pressure and the flow rate of the injectedplastic, for example as described and illustrated in document U.S. Pat.No. 9,102,085 on behalf of the Applicant.

During the molding process there may arise the problem related to theunwanted opening of the mold caused by an overpressure of the plasticmaterial injected into the cavity. Such opening, even though slight, maycause damage if not the breaking of the mold, as well as the formationof clearly visible defects on the molded articles.

In order to overcome this problem, it was proposed to detect anyoverpressure of the plastic material injected into the cavity andcorrect them, or reduce or eliminate them, through the electroniccontrol of the or each injector. Thus, document U.S. Pat. No. 5,556,582proposes to detect any overpressure of the injected plastic material byusing sensors provided in the cavity of the mold. When molding, thedegree of opening of the or of each injector is adjusted based on thedetection carried out by the pressure sensor in the cavity.

This solution, though efficient when it comes to detecting andcorrecting possible overpressures, entails the drawback lying in thefact that the presence—inside the cavity of the pressure sensor orseveral pressure sensors in case of large articles to be molded—entailsmore or less evident defects on the molded pieces.

Document EP 1990603 describes a laser device applied outside a mold fordetecting any separation between the moveable part and the fixed part ofthe mold.

Though solving the technical problem regarding the detection of thepossible opening of the mold, this solution is relatively complex andexpensive given that it requires a precise alignment between the laseremitter, reflector and receiver applied to the two parts of the mold.Furthermore, the possible opening detected by the laser device is simplydisplayed on a display or at most signalled through an acoustic alarm,and thus corrected through the displacement of the re-closure by themoveable part of the mold, thus entailing further technologicalcomplexities.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the technical problemdefined above and such object is attained thanks to the fact that in theinjection molding method according to the invention the detection of theopening of the mold caused by an overpressure of the plastic materialinjected into the mold cavity is carried out by means of transducersprovided outside the cavity of the mold and the detected opening of themold is corrected by adjusting the stroke and/or the speed and/or theopening/closing acceleration of said at least one injector during theinjection step and/or during the subsequent step of packing the injectedplastic material.

The transducer means can be provided outside or inside the mold, butoutside its cavity in any case. For example, the transducer means can bepositioned on the fixed part of the mold, or on the molding machineplate, or on the slide columns of the moveable part of the mold.

The transducer means can be selected from among deformation sensors,flexion sensors, torsion sensors or pressure sensors. Such sensors canbe of the contact type or of the magnetic induction type.

Should the injection be actuated by a plurality of autonomouslycontrolled injectors, the method according to the invention furtherprovides for an auto-tuning method wherein, upon defining the tolerablemaximum opening value of the mold, the opening of an injector iscommanded sequentially at a time and the impact of each injector isverified through the aforementioned transducer means depending on suchmaximum value. The impact of each injector in the course of thesubsequent molding cycle is taken into account during the electroniccontrol.

The invention also regards an injection molding apparatus for theimplementation of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to theattached drawings, provided purely by way of non-limiting example,wherein:

FIG. 1 is a schematic perspective view showing—in a transparent manner—afirst example of an injection molding apparatus for implementing themethod according to the invention,

FIG. 2 is a lateral elevational view of FIG. 1,

FIG. 3 is a top plan view of FIG. 2, and

FIGS. 4, 5 and 6 are views respectively analogous to FIGS. 1, 2 and 3,showing a second example of the apparatus for implementing the injectionmolding method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Initially with reference to FIGS. 1, 2 and 3, a first example of anapparatus for the injection molding of plastic materials for theimplementation of the method according to the invention consists in amold 1 formed by a fixed part or matrix 2, applied in a known and notillustrated manner to a molding machine plate, and a moveable part ordie 3 displaceable with respect to the fixed part 2 along the slidecolumns, also known and not illustrated in detail. In the closingposition of the mold 1 represented in the drawings the fixed andmoveable parts 2, 3 mutually define a molding cavity 4. In the case ofthe illustrated example, the cavity 4 is configured for molding a largespoiler for use in the automotive industry.

The mold 1 is equipped with an injection system which conventionallyincludes a distributor or hot runner 5 supplied by a molding press andconnected to a series of injectors 6 also of the conventional type. Itshould be observed that the number and arrangement of the injectors 6are provided purely by way of example.

Each injector 6 comprises—in a per se known manner and thus notillustrated in detail,—a nozzle in which there is axially mobile a pinvalve displaceable between a position of full closure and a position ofmaximum opening for the injection of the pressurised plastic materialinto the cavity 4. The displacement of the pin valve of each injector 6is controlled by a respective actuator 7, of the fluid or moreconveniently of the electrical type, for example of the rotary type,controlled by means of an electronic unit schematised with 8.

The electronic control unit 8 is programmed so as to control eachactuator 7, and thus each pin valve 6, so as to adjust the pressure andthe flow rate of the plastic material injected by it into the cavity 4during the molding cycle. The control may in particular provide for theadjustment of the position, speed, acceleration and stroke of the pinvalve of the injector 6 during the step of filling the cavity 4.

The molding apparatus according to the invention is also provided withtransducer means provided for detecting an undesired opening of the mold1 caused by an excessive pressure of the plastic material injected intothe cavity 4 and operatively connected to the electronic unit 8 which isprogrammed, as clarified hereinafter, so as to limit or even eliminatethe opening of the mold 1.

According to the distinctive characteristic of the invention, thesetransducer means are positioned outside the cavity 4 and they arearranged inside the mold 1 or even outside the same.

In the embodiment represented in FIGS. 1, 2 and 3 the transducer meansconsist of a series of four sensors 9 positioned in the fixed part 2 ofthe mold 1 so that they face the moveable part 3 in the closing positionof the latter. The sensors 9 can be selected from among deformationsensors, flexion sensors, torsion sensors, pressure sensors, and theycan be of the contact type or of the magnetic induction type.

When operating, should the mold 1 be closed and the plastic materialcoming from the hot runner 5 be introduced into the cavity 4 by means ofthe injectors 6, the sensors 9 detect a possible opening movement of themoveable part 3 relatively to the fixed part 2 due to excessive pressureof the plastic material injected into the cavity 4 and sendcorresponding signals to the electronic unit 8. The electronic unit 8identifies the injector/s 6 that caused the possible detectedoverpressure and corrects it by adjusting the opening stroke and/or thespeed and/or acceleration of the relative pin valve, thus restoringoptimal pressure. Basically, a maximum opening value of the mold 1 ispredefined upon reaching which the opening of the injector/s 6 thattriggered such maximum value is interrupted or reduced.

It should be observed that the aforementioned correction step can beactuated not only in the step of introducing the plastic material intothe mold but also during the subsequent step of packing the injectedplastic material.

In the case of the variant represented in FIGS. 4, 5 and 6, in whichparts identical or similar to the ones already described previously areindicated using the same reference numbers, the sensors 9 are appliedoutside the fixed part 2 and the moveable part 3 of the mold 1, inmutually faced conditions.

It should be observed that the transducers for detecting the undesiredopening of the mold 1 could also be arranged in positions different fromthe ones represented in the illustrated examples, and even outside themold 1 such as for example on the molding machine plate or along theslide columns of the moveable part 3 relatively to the fixed part 2. Inany case, these sensors will always be arranged outside the cavity ofthe mold 1, hence their presence will not affect the aesthetic andstructural characteristics of the articles molded in the cavity 4 in anymanner whatsoever.

As previously mentioned, the invention provides for, at the beginning ofthe molding cycle, an initial auto-tuning step in which, upon defining atolerated maximum opening value of the mold 1, the opening of aninjector 6 is sequentially commanded at a time and there occurs, bymeans of the sensors 9, the impact of each injector depending on thetolerated maximum opening value. This allows detecting which areas ofthe mold 1 are most stressed by each single injector 6, irrespective ofthe opening sequence of the injectors 6 during the molding cycle. Thisallows monitoring the impact of each injector 6 and the electronic unit8 is capable of calibrating the injectors that affect a particular areaof the mold 1.

Obviously, the construction details and the embodiments may widely varywith respect to what has been described and illustrated, withoutdeparting from the scope of protection of the present invention asdefined in the claims that follow.

The invention claimed is:
 1. A method for the injection molding ofplastic material into a cavity of a mold by a plurality of autonomouslycontrolled injectors each having a pin valve whose opening and closingare operated through an electronic control unit, the method comprising:detecting and correcting, by said electronic control unit of said atleast one injector, an undesired opening of the mold caused by anyoverpressure of the plastic material injected into the cavity; whereindetection of said undesired opening of the mold occurs through one ormore transducer arrangements provided outside the cavity of the mold;wherein the detected undesired opening of the mold is corrected byadjusting a speed and/or a closing/opening acceleration of one or moreof said pin valves during injection of the plastic material and/orduring a subsequent packing of the injected plastic material; andwherein a maximum opening value of the mold is established, and uponreaching which the opening of one or more of said pin valves duringtheir opening stroke is interrupted.
 2. The method according to claim 1,wherein the mold includes a fixed part and a movable part, and whereinsaid transducer arrangements are positioned on said fixed part of themold.
 3. The method according to claim 1, wherein said fixed part of themold is applied on a molding machine plate, and wherein said transducerarrangements are positioned on said molding machine plate.
 4. The methodaccording to claim 1, wherein the mold includes a fixed part and amovable part displaceable relative to said fixed part along slidecolumns, and wherein said transducer arrangements are positioned on saidcolumns.
 5. The method according to claim 1, wherein said transducerarrangements are selected among deformation sensors, flexion sensors,torsion sensors, and pressure sensors.
 6. The method according to claim5, wherein said sensors are of a contact type.
 7. The method accordingto claim 5, wherein said sensors are of a magnetic induction type. 8.The method of claim 1 further comprising an initial auto-tuning step ata beginning of a molding cycle in which, upon defining the maximumopening value of the mold: an opening of each pin valve is sequentiallycommanded at a time and there occurs, through said transducerarrangements, a measured impact of each injector's pin valve on theundesired opening of the mold relative to said maximum opening value,based on the measured impact, determining one or more areas of the moldwhere each single injector has the largest measured impact on theundesired opening of the mold relative to said maximum opening value,wherein such impact is monitored during electronic control of the pinvalve of each injector during a subsequent molding cycle, and based onsaid monitoring and measured impact, calibrating one or more injectorsof the plurality of injectors to reduce the measured impact on theundesired opening of the mold relative to the maximum opening value in aparticular area of the mold.